The present invention relates generally to improvements in photovoltaic cathodic protection systems for metal structures, such as oil well casings, and, more particularly, pertains to a new and improved power output control circuit for such protection systems wherein the electrical output of a solar energy converter and the electrical output of a battery are utilized together.
The application of direct current to a metallic structure in an electrolyte, such as soil or water, for example, to prevent loss of metal from the metallic structure due to electrolytic corrosion is a known technique. The process involves essentially the formation of a hydrogen ion film on the metallic structure located in the ground or water. This ion film is produced by the direct current flow. The ion film prevents corrosion of the metallic structure by shielding it from the action of the electrolyte.
The extent of the protective hydrogen ion film formed on the metallic structure, that is, the depth to which it is formed, is a function of the amount of current applied, soil resistivity, circuit resistance, and placement of current-discharging electrodes.
The direct current is normally applied to the metal structure by storage batteries. These batteries are kept charged by an array of solar panels, such as photovoltaic cells, which convert sunlight to electrical energy. The power output of the batteries to the metal structure is controlled by a resistive circuit that prevents excessive discharge of the batteries. This is necessary so that the batteries will not deliver more power to the metal structure than the solar panel array can return to the batteries.
To prevent overcharging of the batteries during periods of peak solar activity, a voltage regulator is used. This regulator provides a resistance circuit across the output of the solar array as the batteries approach full charge. After the batteries are fully charged, they continue to supply a current to the metallic structure. Any excess output of the solar array is dissipated by the resistance circuit in the form of heat. Such a system wastes the output of the solar array after the batteries of the system have been charged.